The present invention relates to a fine pattern forming method according to which an optional resist pattern for high accuracy fine processing is formed on a semiconductor substrate by direct writing with electrically charged beams such as electron beam and focused beam using single-layer or multi-layer resist method and more particularly to a fine pattern forming method which is carried out using a high molecular organic film in which an organometallic complex or a metallic salt is incorporated.
Hitherto, in production of IC and LSI, formation of pattern has been carried out by photolithography using ultraviolet rays. With elements becoming finer, it has been tried to enhance NA of stepper lens or to use a light source of short wavelength, but there is the defect that focus depth becomes smaller. Furthermore, for fining of pattern size of LSI device and production of ASIC, electron beam lithography is used. For fine pattern formation by this electron beam lithography, positive type electron beam resists are essential. Among them, polymethyl methacrylate (PMMA) has been known to be highest in resolution, but this has the defect of low sensitivity. Therefore, recently, there have been made many reports to enhance the sensitivity of positive type electron beam resists and there have been proposed positive type electron resists such as, for example, polybutyl methacrylate, a copolymer of methyl methacrylate and methacrylic acid, a copolymer of methacrylic acid and acrylonitrile, a copolymer of methyl methacrylate and isobutylene, polybutene-1-sulfone, polyisopropenyl ketone, and fluoro polymethacrylate. In these resists, scission of principal chain by electron beam can be easily conducted by introducing electron withdrawing group into side chain or by introducing easily decomposable bond into principal chain and thus enhancement of sensitivity is aimed at, but they still do not fully satisfy both the resolution and sensitivity.
Furthermore, electron beam lithography suffers from the problems such as low dry etch resistance of electron beam resists, influence on the pattern accuracy by proximity effect caused by forward scattering and back scattering of electron and influence on pattern writing due to charging of incident electron. For solving these problems, multi-layer resist method according to which action of resist is divided to pattern forming layer and planerizing layer is very effective method. FIG. 6 is a flow sheet which explains tri-layer resist process in electron beam lithography. On a semiconductor substrate 1, an organic film of 2-3 .mu.m is coated as bottom layer 41 for inhibiting proximity effect, then an inorganic film such as SiO.sub.2 or SOG (spin on glass) is coated thereon as intermediate layer 42, electron beam resist 43 is coated as a top layer and furthermore, aluminum film 44 is vapor deposited thereon at a thickness of about 100 .ANG. for inhibiting charging [FIG. 6A]. After exposure, aluminum layer 44 is removed with an aqueous alkaline solution, followed by development [FIG. 6B]. Then, dry etching of intermediate layer 42 is performed using the resulting resist pattern as a mask [FIG. 6C]. Then, dry etching of bottom layer 41 is performed using the intermediate layer as a mask [FIG. 6D]. By using the above-mentioned tri-resist process, fine pattern of high aspect ration can be formed. It is possible to carry out a simple double-layer resist process by using a resist possessing the actions of both the top layer and the intermediate layer of the above tri-layer resist. FIGS. 7A-7D are a flow chart which explains the double-layer resist process. An organic film is formed as bottom film 51 and a silicon-containing resist 52 high in dry etch resistance is formed as a top layer [FIG. 7A]. After exposure and development [FIG. 7B], the bottom layer is subjected to dry etching using the top layer resist pattern as a mask [FIG. 7C]. In this way, the double-layer resist process can form fine pattern with higher through-put as compared with tri-layer resist process.
However, the silicon-containing resist high in dry etch resistance which is used in the double-layer resist is mainly negative type and is inferior in resolution owing to swelling caused by development and besides is low in sensitivity and hence cannot be practically used. Especially, due to charging effect, as shown by the surface SEM photograph of FIG. 7D, field butting error occurs to cause breaking of pattern. Such breaking of pattern also occurs in the tri-layer resist if the aluminum thin film is not present. That is, according to the electron beam exposure, area A is scanned with electron beam in the manner as shown by arrow O and then area B is scanned in the manner as shown by arrow P. In this case, if charging occurs, the resist pattern after development is broken (field butting error) between areas A and B. The resist patterns in areas A ad B must essentially be in connected state. Moreover, the tri-layer resist process is much complicated in steps and have many problems such as defect and difficulty in control of pattern size and is difficult to put to practice.
Furthermore, in electron beam lithography, it becomes important to form an image reversal pattern. Therefore, at present, image reversal image is formed by using a negative type electron beam resist or by baking or treatment with ammonia vapor after exposure.
However, negative type resist is low in resolution and it is difficult to form fine resist pattern. Further, formation of image reversal pattern by post-treatment after exposure is apt to be affected by the conditions of the post-treatment and control of pattern size is difficult.
As explained above, the double-layer resist process is a very effective method, but has problems in the resists used. That is, the materials must be high in dry etch resistance and act as a resist. Resists used at present as top layer of double-layer resist include organopolysiloxane or organopolysilane type silicon-containing resists. However, these resists are not so high in both the sensitivity and the resolution and besides are not sufficiently high in dry etch resistance.
When image reversal pattern is formed by electron beam lithography, negative type resist is inferior in resolution due to swelling or formation of whisker during development treatment and sensitivity is also not so high. Formation of reversal pattern by the post-treatment after exposure is affected by the conditions of the post-treatment and control of size of pattern is difficult and besides this reversal pattern can be formed only with specific resists. Furthermore, dry etch resistance of positive type electron beam resist is extremely low and etching of substrate cannot be performed with single-layer electron beam resist.